bug#69571: 29.2; csharp-mode indentation: Misaligned closing brace in blocks starting below "new"

[Theodor Thornhill via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Tue, 5 Mar 2024 22:09:51 +0100
>> From: Carlos <carlos@cvkm.cz>
>> 
>> Any block starting on the line immediately below a line having the
>> string "new" will have its closing brace aligned with the opening one.
>> 
>> See the following code:
>> 
>> public class Foo {
>>     void Bar () {
>> 	var x = new X(); // [1]
>> 	for (;;) {
>> 	    x();
>> 		 }  // [2]
>>     }
>> }
>> 
>> Line [1] says "new". The closing brace in line [2] is aligned to the
>> opening brace.
>> 
>> If you comment out the "new" (or the whole line) the problem persists.
>> 
>> If you remove the "new" the problem goes away and [2] is correctly
>> aligned.
>> 
>> If you insert a line between line [1] and the one having the opening
>> brace the problem goes away.
>
> Theo and Yuan, could you please look into this?

I have a working patch for this, but I'd like to expand it to cover an
edge case for which I'm unable to find a good solution.  Can you suggest
a way around this edge case?

Consider the provided code:
```
public class Foo {
    void Bar () {
        var x = new X(); // [1]
        for (;;) {
            x();
        }  // [2]
    }
}
```

Like this, the below patch doesn't work. If you remove the first
comment, the patch works.

```
public class Foo {
    void Bar () {
        var x = new X();
        for (;;) {
            x();
        }  // [2]
    }
}
```

The reason is simple, of course. What I'm struggling with here is how to
best handle the case where there is a comment ending the line, possibly
containing a ';' itself. I've tried some variations with save-excursion
along with syntax-ppss to detect whether or not we're in a comment, but
it gets verbose and ugly. Is there some simple way to do this check in
Emacs, or should I just resort to making some best effort judgement call
here?

Thanks,
Theo

diff --git a/lisp/progmodes/csharp-mode.el b/lisp/progmodes/csharp-mode.el
index 7bf57bcbe21..00278e18e51 100644
--- a/lisp/progmodes/csharp-mode.el
+++ b/lisp/progmodes/csharp-mode.el
@@ -495,9 +495,10 @@ csharp-guess-basic-syntax
        (unless (eq (char-after) ?{)
          (ignore-errors (backward-up-list 1 t t)))
        (save-excursion
-         ;; 'new' should be part of the line
+         ;; 'new' should be part of the line, but should not trigger if
+         ;; statement has already ended, like for 'var x = new X();'.
          (goto-char (c-point 'iopl))
-         (looking-at ".*new.*")))
+         (looking-at ".*new.*[^;]$")))
      ;; Line should not already be terminated
      (save-excursion
        (goto-char (c-point 'eopl))